Testing for Glaucoma-related Structural and Functional Changes

Murray Fingeret, OD, FAAO

Because patients with glaucoma can have structural changes in the optic nerve before visual field loss is apparent, high-resolution imaging of the retinal nerve fiber layer (RNFL) is a key to early detection. Scanning laser polarimetry can discern thinning of the RNFL. Evaluated in conjunction with other testing at our disposal, such as applanation tonometry, corneal pachymetry, fundus photography and gonioscopy, scanning laser polarimetry gives us a portrait of a patient's ocular structure.

However, as important as it is for practitioners to assess anatomic structure, it isn't the only factor used to diagnose and assess glaucoma. Functional evaluation with visual field testing shows us how glaucoma is affecting a patient's vision, in ways that he/she cannot detect.

Combined, structural and functional data � and software that turns these data into constructive analyses and images � help us diagnose glaucoma early.

Key Polarimetry Measures

Scanning laser polarimetry sends polarized light through the RNFL to create high-resolution maps. With over 32,000 data points, we get a picture of how much thinning has occurred.

As the polarized light encounters microstructures in the RNFL, it undergoes a phase shift, and the size of that phase shift is expressed as a measurement called RNFL Integrity (RNFL-I). Adding integrity data to our measurements of RNFL volume is useful for early detection of glaucoma because studies have shown that microstructures in the RNFL can change orientation or density before thinning occurs.1,2

In the GDxPRO report, an RNFL-I summary table provides practitioners with color-coded comparisons of different parameters (TSNIT average, superior average and inferior average) against normative limits. The same table shows another key measure for diagnosing glaucoma: the Nerve Fiber Indicator (NFI). The GDxPRO calculates this as a 0-to-100 index of the likelihood that a patient's RNFL is abnormal based on several RNFL measurements.

In addition to these metrics and the nerve fiber layer map, the GDxPRO report incorporates a color-coded deviation map, which illustrates how regions of the RNFL deviate from the normative database, with the fundus image adjacent for a side-by-side comparison.

Imaging and Tracking

When it comes to imaging, resolution has to go hand in hand with accuracy. In scanning laser polarimetry, a feature called variable corneal compensation (VCC) neutralizes corneal birefringence based on the eye's individual macular retardation.

In addition, enhanced corneal compensation (ECC) in the GDxPRO improves the scan's signal-to-noise ratio. We can take scans of most patients, including high myopes who presented a challenge in the past. Using ECC results in fewer atypical scans than VCC, and the technology's sensitivity and specificity,3-5 allow us to recognize early change. When we use scanning laser polarimetry to track glaucoma progression, several other aspects of the technology are important. Serial images require alignment to register properly. The GDxPRO handles that challenge with an automated image alignment feature that aligns all scans to a reference image based on the positions of retinal blood vessels. It ensures that practitioners get a good set of images over multiple visits and even with multiple operators.

Threshold Testing

Structure and function have a correlation, and we need to look at both to get a complete picture. The two types of visual field testing used most commonly are standard automated perimetry (SAP) and frequency doubling technology (FDT) perimetry. With SAP, patients look at a fixation light while a target is flashed in their peripheral vision. Patients indicate when they see a light, and the technology maps their missed flashes. With FDT perimetry, patients look for a set of vertical flickering bars flashed in the peripheral vision (the frequency never changes, only the contrast). The device records when the patient cannot see the bars and plots a map of the visual thresholds based on that information.

For glaucoma, the Humphrey Matrix® Perimeter (Carl Zeiss Meditec) has some advantages. First, it may be more effective than SAP in detecting early loss. Several studies have shown that FDT detects visual field loss that other tests do not.6-14

The Matrix reports provide either single or serial views for tracking patients over time, and the Glaucoma Hemifield Test analysis software compares the upper and lower hemispheres of the visual field, and then weighs that data against a normative database to determine whether the results are statistically within the normal range. Maps produced by the Matrix allow practitioners to see visual field loss locations, which we can compare side by side with maps of the RNFL.

Dr. Fingeret is Chief of Optometry at the Brooklyn/St. Albans Campus of the Department of Veterans Affairs, New York Harbor Health System. He is a clinical professor at the State University of New York College of Optometry and Executive Vicepresident of the Optometric Glaucoma Society.